We’re on pace for 4°C of global warming. Here’s why that terrifies the World Bank.
Posted by Brad Plumer
on November 19, 2012 at 11:28 am

Over the years at the U.N. climate talks, the goal has been to keep future global warming below 2°C. But as those talks have faltered, emissions have kept rising, and that 2°C goal is now looking increasingly out of reach. Lately, the conversation has shifted toward how to deal with 3°C of warming. Or 4°C. Or potentially more.

And that topic has made a lot of people awfully nervous. Case in point: The World Bank just commissioned an analysis (pdf) by scientists at the Potsdam Institute looking at the consequences of a 4°C rise in global temperatures above pre-industrial levels by 2100. And the report appears to have unnerved many bank officials. “The latest predictions on climate change should shock us into action,” wrote World Bank President Jim Yong Kim in an op-ed after the report was released Monday.

So what exactly has got the World Bank so worried? Partly it’s the prospect that a 4°C world could prove difficult—perhaps impossible—for many poorer countries to adapt to. Let’s take a closer look at the report:

1) The world is currently on pace for around 3°C to 4°C of global warming by the end of the century. In recent years, a number of nations have promised to cut their carbon emissions. The United States and Europe are even on pace to meet their goals. But those modest efforts can only do so much, especially as emissions in China and India keep rising. Even if all current pledges get carried out, the report notes, ”the world [is] on a trajectory for a global mean warming of well over 3°C.” And current climate models still suggest a 20 percent chance of 4°C warming in this emissions scenario.

2) The direct consequences of a 4°C rise in global temperatures could be stark. Four degrees may not sound like much. But, the report points out, the world was only about 4°C to 7°C cooler, on average, during the last ice age, when large parts of Europe and the United States was covered by glaciers. Warming the planet up in the opposite direction could bring similarly drastic changes, such as three feet or more of sea-level rise by 2100, more severe heat waves, and regional extinction of coral reef ecosystems.

3) Climate change would likely hit poorer countries hardest. The World Bank focuses on poverty reduction, so its climate report spends most of its time looking at how developing countries could struggle in a warmer world. For instance, a growingnumber of studies suggest that agricultural production could take a big hit under 3°C or 4°C of warming. Countries like Bangladesh, Egypt, Vietnam, and parts of Africa would also see large tracts of farmland made unusable by rising seas. “It seems clear,” the report concludes, “that climate change in a 4°C world could seriously undermine poverty alleviation in many regions.”

4) Yet the effects of 4°C warming haven’t been fully assessed — they could, potentially, be more drastic than expected. Perhaps the most notable bit of the World Bank report is its discussion of the limits of current climate forecasts. Many models, it notes, make predictions in a fairly linear fashion, expecting the impacts of 4°C of warming to be roughly twice as severe as those from 2°C of warming. But this could prove to be wrong. Different effects could combine together in unexpected ways:

Quote:

For example, nonlinear temperature effects on crops are likely to be extremely relevant as the world warms to 2°C and above. However, most of our current crop models do not yet fully account for this effect, or for the potential increased ranges of variability (for example, extreme temperatures, new invading pests and diseases, abrupt shifts in critical climate factors that have large impacts on yields and/or quality of grains).

What’s more, the report points out that there are large gaps in our understanding of what 4°C of warming might bring: “For instance,” it notes, “there has not been a study published in the scientific literature on the full ecological, human, and economic consequences of a collapse of coral reef ecosystems.”

5) Some countries might not be able to adapt to a 4°C world. At the moment, the World Bank helps many poorer countries build the necessary infrastructure to adapt to a warmer world. That includes dams and seawalls, crop research, freshwater management, and so forth. But, as a recent internal review found, most of these World Bank efforts are focused on relatively small increases in temperature.

This new World Bank report is less sure how to prepare for a 4°C world. “[G]iven that uncertainty remains about the full nature and scale of impacts, there is also no certainty that adaptation to a 4°C world is possible.” That’s why, the report concludes, “The projected 4°C warming simply must not be allowed to occur — the heat must be turned down. Only early, cooperative, international actions can make that happen.”

So what sorts of actions might that entail? The International Energy Agency recently offered its own set of ideas for curbing greenhouse-gas emissions and keeping future warming below 2°C. That included everything from boosting renewable energy to redesigning the world’s transportation system. But so far, nations have only made small progress on most of these steps.

According to a new Berkeley Earth study released today, the average temperature of the Earth’s land has risen by 1.5 °C over the past 250 years. The good match between the new temperature record and historical carbon dioxide records suggests that the most straightforward explanation for this warming is human greenhouse gas emissions.

The new analysis from Berkeley Earth goes all the way back to 1753, about 100 years earlier than previous groups’ analyses. The limited land coverage prior to 1850 results in larger uncertainties in the behavior of the record; despite these, the behavior is significant. Robert Rohde, Lead Scientist for Berkeley Earth and the person who carried out most of the analysis, noted that “Sudden drops in the early temperature record (1753 to 1850) correspond to known volcanic events.” Volcanoes spew particles into the air, which then reflect sunlight and cool the earth for a few years. In the Berkeley Earth temperature plot (see figure below), sudden dips in temperature caused by large volcanic explosions are evident back to the late 1700s.

Figure: The temperature of the Earth’s land surface, as determined from over 36,000 temperature stations around the globe. The data is well fit by a simple model containing only known volcanic eruptions and carbon dioxide (dark line). No contribution from solar variability was necessary to make a good match. The rapid but short (decadal) variations are believed to be due to changes in ocean flows, such as El Nino and the Gulf Stream.

Berkeley Earth compared the shape of the gradual rise over 250 years to simple math functions (exponentials, polynomials) and to solar activity (known through historical records of sunspot numbers), and even to rising functions such as world population.

Richard Muller, Founder and Scientific Director of Berkeley Earth, notes “Much to my surprise, by far the best match was to the record of atmospheric carbon dioxide, measured from atmospheric samples and air trapped in polar ice.” He emphasizes that the match between the data and the theory doesn’t prove that carbon dioxide is responsible for the warming, but the good fit makes it the strongest contender. “To be considered seriously, any alternative explanation must match the data at least as well as does carbon dioxide.”

In its 2007 report the IPCC concluded only that “most” of the warming of the past 50 years could be attributed to humans. It was possible, according to the IPCC, that increased solar activity could have contributed to warming prior to 1956. Berkeley Earth analyzed about 5 times more station records than were used in previous analyses, and this expanded data base along with its new statistical approach allowed Berkeley Earth to go about 100 years farther back in time than previous studies. By doing so, the Berkeley Earth team was able to conclude that over 250 years, the contribution of solar activity to global warming is negligible.

Some of the scientists on the Berkeley Earth team admit surprise that the new analysis has shown such clear agreement between global land-*‐temperature rise and human-*‐caused greenhouse gases. “I was not expecting this,” says Richard Muller, “but as a scientist, I feel it is my duty to let the evidence change my mind.”

Elizabeth Muller, co-*‐Founder and Executive Director of Berkeley Earth, says that “One of our goals at Berkeley Earth is complete transparency – we believe that everyone should be able to access raw climate data and do their own analysis. Scientists have a duty to be ‘properly skeptical’, and we are trying to lower the barriers to entry into the field.”

Robert Rohde created an online feature that allows people to look up temperature records by location. “If you want to know what the temperature change has been in your city, your state, or even your country, you can now find this online at BerkeleyEarth.org” says Rohde. He adds, “We hope people will have a lot of fun interacting with the data.” This feature should be available to the public by Monday July 30.

A previous Berkeley Earth study, released in October 2011, found that the land-*‐surface temperature had risen by about 0.9 °C over the past 50 years (which was consistent with previous analyses) and directly addressed scientific concerns raised by skeptics, including the urban heat island effect, poor station quality, and the risk of data selection bias.

The Berkeley Earth team values the simplicity of its analysis, which does not depend on the large complex global climate models that have been criticized by climate skeptics for their hidden assumptions and adjustable parameters. The conclusion that the warming is due to humans is based simply on the close agreement between the shape of the observed temperature rise and the known greenhouse gas increase.

Elizabeth adds, “The current data does not include ocean temperatures; these will be added in the next phase of the Berkeley Earth studies. Another next step for our team is to think about the implications of our findings.”

This shows that while scientists cannot prove with certainty how much of this is anthropogenic, the correlation between human CO2 output and global temp is pretty damning...

The annual and decadal land surface temperature from the BerkeleyEarth average, compared to a linear combination of volcanic sulfate emissions and the natural logarithm of CO2. It is observed that the large negative excursions in the early temperature records are likely to be explained by exceptional volcanic activity at this time. Similarly, the upward trend is likely to be an indication of anthropogenic changes. The grey area is the 95% confidence interval.